Implementation of grain mapping by diffraction contrast tomography on a conventional laboratory tomography setup with various detectors

Implementation of grain mapping by diffraction contrast tomography has been demonstrated on a conventional tomography setup with two common detectors (CCD and flat panel). Typical grain mapping performance has been characterized for the related experimental conditions and setup, and grain reconstructions from diffraction images acquired with different exposure times demonstrate the possibility of fast grain mapping with laboratory-based X-rays.

Diffraction projections have been acquired on the AlCu alloy sample to study the effects of several key parameters, including sample-to-detector distance (Lsd), source voltage and source size, on the quality of the projection, thereby providing a guideline for choosing a proper combination of different experimental parameters for LabDCT experiments using the current tomography instrument.
Other parameters, such as sample-to-source distance (Lss), exposure time, pinhole position and size and beamstop used for the CCD and flat panel detectors, were kept constant.
It is concluded that a source voltage of 45 -60 kV using the nano source in a middle or a large size mode are favorable to obtain high quality LabDCT projections for this sample using the present tomography instrument. Given the same pinhole and Lss as described in the paper, Lsd is suggested to be in the range 195 -250 mm for the flat panel and 50 -65 mm for the CCD by considering the background noise level, probability of spot overlap and geometric constraints of the instrument. The results are presented as follows together with additional results of LabDCT projections measured with the CCD detector at shorter exposure times compared to the result presented in the paper. Notably, all the images here are shown in log scales to enhance the visibility of diffraction spots in visualizations.

Diffraction projection as a function of sample-to-detector distance
For the flat panel, LabDCT diffraction projections were acquired with an exposure time of 4 s and Lss ≈ 9.2 mm as a function of Lsd in the range 375 -195 mm, with the latter distance close to the geometric constraint of this setup. As shown in Figure S1, the diffraction spots on the images become smaller and at the same time the area affected by high background intensity (caused by scattering & partial transmission from edges of the pinhole) shrinks with decreasing Lsd. Visual inspection indicates that the diffraction spots are sharper and more evident from the background at smaller Lsd, such as at 225 mm shown in Figure S1c. On the other hand, with decreasing Lsd the probability of diffraction spots overlap increases and hence it tends to form a more crowded spot image (see Figure S1d).  Figure S2 for the diffraction projections measured with Lsd = 95 -51 mm by the CCD detector with an exposure time of 60 s and Lss ≈ 9.2 mm. The diffraction spots are more magnified at longer Lsd, but suffer more harm from the shadow of the direct beam, while at shorter Lsd the spots are more distinct from the background but more easily overlapped with others. To have a quantitative comparison, the average and standard deviation of the background intensity for 8 selected regions in the middle and corner parts of the LabDCT projection (see Figure S3 for their locations) were computed. One may think that the best way to compare the projection quality is to compute the contrast-to-noise ratio of each spot on the projections. However, it should be noted that the same spot across different Lsd appears at different locations of the projections (e.g. a spot on the corner deems to be less visible than a spot in the middle region because of inhomogeneity in background noise) or even disappears (not able to track the same spot all the time). Therefore, here we only focus on quantifying the background intensities in each region (that are selected to be relatively large to minimize the contribution of the spot intensities) to obtain a guide for evaluating the projection quality. This leads to strong rise of / ̅ with increasing Lsd for these regions ( Figure S4c). Whilst the magnitude is much weaker, the ratios for some corner regions are also increasing with increasing Lsd. Figure S4 suggests that keeping Lsd = 195 -250 mm gives a relatively low background intensity as well as a low background noise level.

Diffraction projection as a function of source voltage
LabDCT diffraction projections were acquired using the flat panel with an exposure time of 4 s, Lss ≈ 9.2 mm, Lsd ≈ 225 mm and nano source in a middle size mode with different source accelerating voltage. Figure S6 shows that the contrast over noise ratio of the diffraction spots decreases with increasing source voltage, especially for the two higher values. As forward simulation (including attenuation, source spectrum and detector efficiency, Fang et al., 2020) indicates that most of the exploitable diffraction signal is expected in the range 15 -45 keV for this AlCu alloy sample, voltages in the range between 45keV and 60 kV can be considered optimal for the sample studied here.

Diffraction projection as a function of source size
Diffraction projections were acquired on the flat panel detector (exposure time 4 s, Lss ≈ 9.2 mm and Lsd ≈ 225 mm), using different source size modes of the nano source. Figure S7 shows merely visible spots for the small source, corresponding to 10.2 µA current on the anode target (proportional to the total photon flux). The projections look very similar between the middle (30.9 µA) and the large (50.6 µA) source size. Considering a bigger source might lead to geometric blurring of the diffraction spots and thus deteriorate the spatial resolution in the reconstructed grain map, we cautiously chose middle size mode for the tomographic acquisitions presented in the current study. Since no geometric blurring is noticeable in Figure S7, we conclude that some additional improvements may be achieved by performing future experiments in large source mode of the instrument.  Figure S8 shows LabDCT diffraction projections measured with the CCD detector as a function of exposure time at Lss ≈ 9.2 mm and Lsd ≈ 55.4 mm (the same distances as listed in Table 1 in the paper).

Diffraction projection as a function of exposure time for the CCD detector
The figure shows very limited number of weak diffraction spots for 4 s ( Figure S8a). The number of visible diffraction spots dramatically increases with exposure time increasing to 10 s, for which a high background noise level presents ( Figure S8b). However, the diffraction images become smooth and the background noise level does not change significantly when the exposure time increases to 20 s and onwards.